Cracking of petroleum fractions over acid catalysts represent the most widely used means of molecular weight reduction in refining processes. In a cracking process, in the presence of steam, a potentially effective catalyst in the process is required to have components which retain a high level of acid cracking activity in the presence of steam.
Most of the conventional acid catalysts used in catalytic cracking processes are known to be unsuited for use in the presence of steam at temperatures greater than about 1400.degree. F. (760.degree. C). The effects of water are considered to be those of a structural poison and result in a weakening of the acid strength of the acid sites. Stability in high steam environments is desirable however, since one way of removing the coke which is inevitably deposited on the catalyst is to steam gasify the coke on the catalyst. The burning off of the coke necessarily requires the presence of steam.
In cracking processes it is generally the object to generate lower molecular weight, but commercially valuable products, from higher molecular weight crude feedstocks. The catalysts used in these acid catalyzed processes, however normally suffer from extreme sensitivity to contaminants in the feedstocks such as sulfur, nitrogen and traces of various metals such as nickel, vanadium, iron, chromium. The catalysts are also susceptible to rapid deactivation by the depositing of coke.
U.S. Pat. No. 3,649,707 teaches a multistep, multicatalyst dealkylation using steam. The first reaction zone utilizes a catalyst composite containing as active ingredients a cobalt component, a nickel component or a Group VIB component, preferably composited with a high surface area refractory inorganic oxide type carrier characterized in having a surface area of at least 50 m.sup.2 /gm. These carriers include alumina, silica, silica-alumina, zirconia, thoria, magnesia, titania, bauxite, mordenite, faujasite, etc. The cobalt, nickel or Group VIB component can be in the element or preferably the oxide form. This catalyst is used in a first reaction zone for dealkylating an alkyl aromatic at dealkylation conditions controlled to produce a first reaction zone effluent wherein about 1 to about 20 wt. % preferably about 1 to 10 wt. % of the alkylaromatic passed to said zone is dealkylated. Zone one is run at conditions which include a temperature of about 300.degree. C. to about 700.degree. C., a pressure of about atmospheric to about 100 atmospheres, a steam to hydrocarbon mole ratio of about 1:1 to about 30:1 or more and a liquid hourly hydrocarbon space velocity of about 0.1 to about 20 hr.sup.-1 or more. The reaction practiced in this zone is dealkylation as has been previously stated. Steam dealkylation is a catalyzed reaction wherein water is a consumed reactant. This is to be contrasted with acid cat cracking which is associated with acid sites found on the catalyst and which though preferably practiced in the presence of steam does not consume the steam that is, water is not a reactant.
U.S. Pat. No. 2,849,383 teaches catalysts for use in hydrocarbon conversion processes and the processes themselves, wherein the catalyst consists essentially of a siliceous cracking catalyst such as silica-alumina and an oxide of a metal from Group VB or VIB, V, Nb, Ta, Cr, Mo, W. The siliceous cracking catalyst is described as a synthetic or natural siliceous composition which contains at least 50% silica (calculated as SiO.sub.2). The siliceous material can be silica-alumina, silica-magnesia, silica-zirconia and silica-alumina-zirconia. The metal oxide (from Groups VB or VIB) must be incorporated with the cracking catalyst in a quantity of at least 2.9% by weight. The material can be deposited on the carrier or coprecipitated with the siliceous carrier. It is further stated that an essential step in the preparation of the catalyst composition is the steaming of the composition after calcination for from 1 to 8 hours at temperatures of from 600.degree. C. to 800.degree. C. in 100% steam. While this reference recites the above process in general, there are examples in the specification only of chromia on silica-alumina. The fact that this steaming is effective only for certain combinations of materials when the support is alumina was not recognized by the art.